Polymers grafted onto a metal oxide surface, method of grafting polymers onto a metal oxide surface, graft polymer suitable for the method

ABSTRACT

Metal oxide having a surface onto which a multitude of individual polymers are grafted, each polymer comprising an addition polymer having a first and a second end, and a first moiety comprising a terminal phosphonate group, which first moiety is bonded to the first end, which phosphonate group attaches to the metal oxide surface in such a way that the multitude of the grafted polymers comprises at least one group of adjacent polymers that have a stretched chain conformation wherein the adjacent stretched chains have a substantially parallel orientation, such that the polymers within said group together form a brush structure. Method of grafting a multitude of individual polymers onto a surface of a metal oxide.

This application is a divisional of U.S. application Ser. No. 15/579,274filed Dec. 4, 2017, which is a national phase of InternationalApplication No. PCT/NL2016/050393 filed Jun. 2, 2016 and published inthe English language, which claims priority to Dutch patent applicationno. NL 2014928 filed Jun. 5, 2015.

The present invention relates in a first aspect thereof, to a metaloxide having a surface onto which a multitude of individual polymers aregrafted, each polymer comprising an addition polymer having a first anda second end, and a first moiety comprising a terminal attaching group,which first moiety is bonded to the first end, which terminal attachinggroup attaches to the metal oxide surface.

In a second aspect, the invention relates to a method of graftingpolymers onto a metal oxide surface. In a third aspect, the inventionrelates to a graft polymer suitable for the above method.

In the art, it is well-known to graft polymers onto a metal oxidesurface, in order to impart a functionalization on the metal oxidesurface which is different than the character of the original metaloxide surface itself.

For instance, the strongly polar groups on the metal oxide surface maybe shielded by grafted polymers having a substantially organiccharacter, so that a surface is rendered with a substantially nonpolarcharacter.

An essential requirement for a satisfactory shielding effect, is thatthe polymers are covering the metal oxide surface to such an extent thatthe original metal oxide surface is virtually not influencing thecharacter of the outer layer anymore.

Basically, two methods are used to adhere polymers to a metal oxidesurface: ‘grafting onto’ and ‘grafting from’. The grafting ontotechnique involves the formation of the polymer first, and applying thepolymer in solution onto the metal oxide surface and allowing a terminalattaching group of the polymer to attach to the surface.

The grafting from technique involves the formation of a pre-polymercontaining a terminal attaching group, applying the pre-polymer insolution onto the metal oxide surface, and allowing the attaching groupto attach to the surface. Subsequently a polymer is grown from thepre-polymer up to its desired length.

When using a terminal attaching group such as a carboxyl group,different results have been obtained when using the two differentmethods. The grafting onto technique uses pre-fabricated polymers, whichin solution have the configuration of a random coil. During theattaching process to the metal oxide surface via the attaching group,the polymer chains remain in random coil configuration. In contrast,when using the grafting from technique it has been found that thedistance between neighbouring grafted polymers is considerably smaller,and that the polymers grown from the attached pre-polymer have astretched chain conformation, such that the polymers within said grouptogether form a brush structure composed of parallel oriented, free tailends of adjacent grafted polymers.

The brush structure is very attractive from a point of view of impartinga new functionality to the metal oxide surface, because it virtuallycompletely shields off the original metal oxide surface. However, usingthe grafting from technique is more cumbersome and expensive than thegrafting onto technique.

The objective of the present invention to develop a grafting ontoprocess that is capable in achieving a brush structure of graftedpolymers onto a metal oxide so that the imparted new functionality onthe metal oxide is highly effective.

In the context of the present invention, the term ‘grafted onto’ thusalso encompasses the expression ‘obtained by grafting onto’.

In order to achieve the above objective, the invention relates to

a metal oxide having a surface onto which a multitude of individualpolymers are grafted, each polymer comprising an addition polymer havinga first and a second end,

and a first moiety comprising a terminal phosphonate group, which firstmoiety is bonded to the first end,

which phosphonate group attaches to the metal oxide surface in such away that the multitude of the grafted polymers comprises at least onegroup of adjacent polymers that have a stretched chain conformationwherein the adjacent stretched chains have a substantially parallelorientation, such that the polymers within said group together form abrush structure.

The invention uses phosphonate as a terminal attaching group of thegraft polymer for accomplishing an attachment onto the metal oxidesurface.

Using phosphonate as a terminal attaching group for a graft polymer hasbeen described by Basuki et al. in Macromolecules, 46, 7043-7054, ACSPublications 2013, wherein super-paramagnetic iron oxide nanoparticlesof 10 nm are grafted with a block copolymer. Between the free tail endsof the grafted polymers large intermediate spaces are present in whichionic dyes are to be encapsulated. These special nanoparticles areapplied in biomedical research. Because of their relatively small size,the nanoparticles are to be regarded as small spheres whichintrinsically are not suitable for the formation of a brush structure,as encompassed by the invention.

In particular, the metal oxide according to the invention includes thefeature that the group of polymers forming a brush structure, aregrafted onto a non-spherical metal oxide surface.

Such surfaces, in particular planar surfaces and the like, promote thestretched chain conformation because the free tail ends of adjacentgrafted polymers have a limited distance to each other which is more orless equal to the distance between the attaching phosphonate groups.

The metal oxide according to the invention includes various metal types,such as titanium, iron, vanadium, cerium, tungsten, copper and antimony.

It is preferred that the metal oxide according to the invention includesthe feature that the metal oxide is diamagnetic. This feature isadvantageous in view of the intended applications of the grafted metaloxide surfaces, wherein it is in general to be avoided that the surfaceshave a paramagnetic behaviour.

It is especially preferred that the metal oxide according to theinvention is titanium dioxide.

According to a preferred embodiment of the metal oxide according to theinvention, the group of polymers forming a brush structure, have anaverage distance D on the metal oxide surface between adjacent polymers,wherein D/2 is smaller than the average radius of gyration Rg of arandom coil conformation of the individual grafted polymers.

It has been found that such values further promote the formation ofstretched chains of grafted polymers.

Furthermore, it is preferred in the metal oxide according to theinvention, that the the grafted polymers have a small polydispersityindex (PDI) which is the ratio of the weight average molecular weight(Mw) and is the number average molecular weight (Mn), the PDI valuebeing between 1 and 2, more preferably between 1 and 1.5.

Such grafted polymers have a more uniform size of the free tail ends,which enhances the quality of the obtained brush structure.

It is especially preferred in the metal oxide according to theinvention, that each of the grafted polymers has a second moietycomprising a terminal aliphatic group, which second moiety is bonded tothe second end via a thiocarbonylthio (—SC(═S)—) group, preferably via atristhiocarbonate (—SC(═S)S—) group.

The above thio-groups are typical for a so-called RAFT reagent, whichallows for a so-called reversible addition fragmentation chain transfer.The RAFT reagent acts as a pre-polymer molecule which allows the growthof a polymer chain next to the thio-group. Advantageously, the RAFTreagent is capable of controlling the extent of polymerization, so thata polymer chain is formed having a relatively low polydispersity.

Particularly preferred is that the terminal aliphatic group is a lineargroup chosen from n-butyl up to n-dodecyl.

It is furthermore preferred in the metal oxide according to theinvention, that the addition polymer comprises a linear chain of carbonatoms, which preferably comprises 10 to 50 carbon atoms. Such length hasbeen found to allow the addition polymer to adapt a stretchedconformation, while being effective in imparting a new functionality tothe metal oxide surface.

In the metal oxide according to the invention, preferably the additionpolymer is a polystyrene, polyisoprene, polyacrylonitrile, polyacrylate,polymethacrylate, ABS, SAN, or a combination thereof.

ABS and SAN are copolymers of respectively acrylonitrile butadienestyrene, and styrene acrylonitrile.

Other suitable polymers that can be used as addition polymer in theinvention are: polyethylene, polypropylene, polyvinyl chloride,polyvinyl acetate, acrylic polymers, polymethyl methacrylate, HEMA andcyanoacrylate polymers, polymethyl and polyethyl acrylates, fluorinatedpolymers such as polytetrafluoroethylene, fluoroelastomers, polyvinylfluoride and polyvinylidene fluoride, diene polymers such aspolybutadiene and polyisoprene, polychloroprene. Also combinations ofany of the above polymers are included as embodiments of the additionpolymer in the context of the invention.

It is preferred in the metal oxide according to the invention, that thephosphorus atom P of phosphonate group is bonded to the first end via acarboxylate group, preferably via a methylene carboxylate group(P—CH2-O(O═)C—).

In particular, it is preferred that the carboxylate group is bonded tothe first end via a —C(CH3)(CH3)- group.

These types of covalent bonding of the phosphonate group to the firstend of the polymer, were found to be successful in acquiring the desiredeffect of the invention.

In particular, it is preferred that the metal oxide according one to theinvention is in the form of a particle, preferably in the range of 20 to200 nm. This size was found to have sufficient surface area to permitthe formation of brush structures of grafted polymers onto the surface.

This size is partly within the IUPAC definition of nanoparticles (up to100 nm), but also includes sizes larger than nanoparticles.

Especially preferred is rutile—a TiO2 crystal—as a nanoparticle, whichhas a tetragonal, ditetragonal, dipyramidal crystal symmetry.

Another embodiment of the invention relates to a metal oxide accordingto the invention, wherein the metal oxide is in the form of amacroscopic sheet, such that the grafted polymers form a coating layer.

The polymers grafted onto the metal oxide surface thus provide aprotective coating against any unwanted effects from the environment,including corrosion.

Furthermore, the polymers grafted onto the metal oxide surface mayfunction as a primer layer to apply a paint layer upon. As such, thepaint layer will adhere better than when applied directly onto the metaloxide surface without the presence of a grafted polymer layer.

In a special first aspect, the invention relates to a polymer materialcontaining a polymer medium in which a multitude of metal oxidesparticles according to the above preferred embodiment are dispersed, andwherein preferably the polymer medium is compatible with the polymersgrafted onto the metal oxides, or more preferably the polymer of themedium is the same as the polymer included in the grafted polymers.

A further preferred embodiment of the invention relates to a dielectricarticle comprising a polymer material according to the above specialaspect, having a relative permittivity ε_(r) of 3 or higher, preferablybetween 5 and 30, more preferably between 10 and 20.

Such dielectric articles have a substantially raised permittivitycompared to the polymer medium itself, which is advantageous in terms ofproducing smaller dielectric articles for electronic equipment,including an antenna.

The dielectric article can for instance be produced by direct mixing ofthe particles with the polymer medium in a compounding machine, oralternatively by suspending the particles in a solution containing thepolymer medium.

In a second aspect, the invention relates to a method of grafting amultitude of individual polymers onto a surface of a metal oxide,wherein each polymer comprises an addition polymer having a first and asecond end, and a first moiety comprising a terminal phosphonate group,which first moiety is bonded to the first end, comprising the steps of:

-   -   dissolving the individual polymers in an appropriate solvent;    -   applying the formed solution onto the surface of the metal        oxide;    -   allowing the individual polymers to attach to the surface,        wherein the phosphonate group attaches to the metal oxide        surface in such a way that the obtained multitude of grafted        polymers onto the surface of the metal oxide comprises at least        one group of adjacent polymers that have a stretched chain        conformation wherein the adjacent stretched chains have a        substantially parallel orientation, such that the polymers        within said group together form a brush structure.

Subsequent to the attachment of the graft polymers onto the surface, thesurface may be dried by allowing the solvent to evaporate. In the caseof particles of metal oxide, the surface of the particles may be driedfurther in a centrifuge.

As explained above in respect of the first aspect of the invention, theknown methods of grafting polymers onto metal oxide surfaces sufferedfrom the drawback that the individual grafted polymers had a random coilconformation. Consequently, a brush structure of adjacent graftedpolymers was not obtainable.

However, it was found that in using a terminal phosponate group on theindividual polymers, which group attaches to the metal oxide surface, itwas possible to achieve a stretched conformation of adjacent graftedpolymers, wherein these polymers together form a brush structure.

Preferred embodiments of the method include the features which arealready described above in view of the first aspect of the invention,and in particular relate to:

-   -   the group of polymers that form a brush structure is being        grafted onto a non-spherical metal oxide surface;    -   the metal oxide used is diamagnetic, preferably titanium        dioxide.

Preferably in the method according to the invention, the group ofpolymers forming a brush structure, have an average distance D on themetal oxide surface between adjacent polymers, wherein D/2 is smallerthan the average radius of gyration Rg of a random coil conformation ofthe individual grafted polymers.

Further preferred embodiments of the method include the followingindependent features:

-   -   the grafted polymers have a small polydispersity index (PDI)        which is the ratio of the weight average molecular weight (Mw)        and is the number average molecular weight (Mn), preferably the        PDI is between 1 and 2, more preferably between 1 and 1.5;    -   each of the grafted polymers has a second moiety comprising a        terminal aliphatic group, which second moiety is bonded to the        second end via a thiocarbonylthio (—SC(═S)—) group, preferably        via a tristhiocarbonate (—SC(═S)S—) group;    -   the addition polymer comprises a linear chain of carbon atoms,        which preferably comprises 10 to 50 carbon atoms;    -   the terminal aliphatic group is a linear alkyl group chosen from        n-butyl up to n-dodecyl;    -   the addition polymer is a polystyrene, polyisoprene,        polyacrylonitrile, polyacrylate, polymethacrylate, ABS, SAN, or        a combination thereof; other suitable polymers or mixtures        thereof are already indicated above in respect of the first        aspect of the invention, and are referred to.    -   the phosphorus atom P of the phosphonate group is bonded to the        first end via a carboxylate group, preferably via a methylene        carboxylate group (P—CH2-O(O═)C—);    -   the carboxylate group is bonded to the first end via a        —C(CH3)(CH3)- group;    -   the metal oxide is in the form of a nanoparticle, preferably in        the range of 20 to 200 nm. Especially preferred is rutile as a        nanoparticle (having a tetragonal, ditetragonal, dipyramidal        crystal symmetry).

In a third aspect, the invention relates to a graft polymer suitable forgrafting onto a metal oxide surface using the method according to thesecond aspect of the invention,

wherein the polymer comprises a linear chain of polyethylene having afirst and a second end, and a first moiety comprising a terminalphosphonate group, which first moiety is bonded to the first end,

wherein the polymer has a second moiety comprising a terminal aliphaticgroup, which second moiety is bonded to the second end via atristhiocarbonate (—SC(═S)S—) group,

wherein the phosphorus atom P of the phosphonate group is bonded to thefirst end via a methylene carboxylate group (P—CH2-O(O═)C—),

and preferably the carboxylate group is bonded to the first end via a—C(CH3)(CH3)- group.

This specific group of graft polymers was found to be suitable to begrafted onto particles of a size in the range of 20 to 200 nm, whileobtaining at least one group of adjacent grafted polymers that have astretched chain conformation, such that the polymers within said grouptogether form a brush structure.

The terminal phosphonate group may be provided with leaving orprotecting groups, e.g. in the form of a phosphonic acid or a bismethylphosphonate.

Preferably, in the graft polymer according to the invention, theaddition polymer comprises a linear chain of carbon atoms, whichpreferably comprises 10 to 50 carbon atoms.

A preferred graft polymer according to the invention includes thefeature that the addition polymer is a polystyrene, polyisoprene,polyacrylonitrile, polyacrylate, polymethacrylate, ABS, SAN, or acombination thereof. Other suitable polymers or mixtures thereof arealready indicated above in respect of the first aspect of the invention,and are referred to.

In particular it is preferred in the graft polymer according to theinvention, that the terminal aliphatic group is a linear alkyl groupchosen from n-butyl up to n-dodecyl.

EXAMPLE

The invention is further illustrated by the below example, together withappended drawings, wherein:

FIG. 1 shows a reaction scheme for producing a graft polymer accordingto the invention;

FIG. 2 shows schematically two adjacent grafted polymers attached to aflat metal oxide surface;

FIGS. 3 a and 3 b show two different conformations of two differentpolymers grafted onto a flat metal oxide surface;

FIG. 4 shows a metal oxide particle of rutile that is preferably used inthe invention.

FIG. 1 shows a reaction scheme wherein a pre-polymer IV is prepared,which has the functionality of a RAFT reagent. The pre-polymer IV isallowed to react with styrene in AIBN and DMF, thus forming a polymer V,which is a graft polymer according to the invention. The graft polymer Vwas prepared in three batches with three different numbers of repeatingstyrene units, wherein n=18, 23 or 42. Pre-polymer IV is prepared byallowing compound I to react with oxalyl chloride in DMF and DCM, thusobtaining compound II. Compound II is reacted with a dimethylphosphonate IIa to obtain compound III. Compound III is reacted inSiMe3Br, DCM, and MeOH, to obtain compound IV.

The graft polymer V (having n=18, 23 or 42) was dissolved in anappropriate solvent such as DMF and brought in a reactor containingrutile nanoparticles. The solution of graft polymer V was allowed toreact with the rutile nanoparticles under ambient conditions for 24hours. Subsequently the particles grafted with polymer V were separatedby centrifuge and dried at 60° C. under reduced pressure.

FIG. 2 shows schematically two adjacent graft polymers 1 that areattached to a flat titanium dioxide surface 3. The distance D on thetitanium dioxide surface between the two adjacent graft polymers 1 isindicated, as well as the average radius of gyration Rg of a random coilconformation of the individual grafted polymers (the polymers are notvisualized as a random coil, but simply as rods perpendicular to thesurface). When D/2 is equal or larger than Rg, the conformation of arandom coil is preferred for reasons of entropy.

FIG. 3 a resp. 3 b show two conformations of two different polymers 4,resp. 5, that are grafted onto a flat titanium dioxide surface 3. Thepolymers 4, resp. 5, were grafted onto the surface 3 by applying asolution of the respective polymers onto rutile nanoparticles accordingto above outlined procedure.

FIG. 3 a shows three adjacent grafted polymers 4, which are almost equalto graft polymer V, with the exception that the terminal attaching groupis not a phosphonate group but a carboxyl group instead. Each graftpolymer 4 has 23 styrene units. The distance D between adjacent polymersis such that D/2 is larger than Rg (the average radius of gyration Rg ofa random coil conformation).

From FIG. 3 a it follows that when grafting onto a metal oxide a polymerhaving as a terminal attaching group a carboxyl group, a random coilconformation is achieved.

FIG. 3 b shows eight adjacent grafted polymers 5, which are exactlyequal to graft polymer V. Each graft polymer 5 has 23 styrene units. Thedistance D between adjacent polymers is such that D/2 is smaller than Rg(the average radius of gyration Rg of a random coil conformation).

FIG. 3 b shows that when grafting onto a metal oxide a polymer having aphosphonate group as a terminal attaching group, a stretched chainconformation is achieved wherein the adjacent stretched chains have asubstantially parallel orientation, such that the polymers within saidgroup together form a brush structure.

The above qualitative difference is supported by the below measurementof the distance D between adjacent polymers grafted onto a flat titaniumdioxide surface:

Attaching group of graft polymer, number of styrene Conformation unitsD/2 (nm) Rg (nm) of polymer chain Carboxylate, 23 1.55 1.51 Random coilPhosphonate, 18 0.74 1.30 Stretched chain Phosphonate, 23 0.79 1.51Stretched chain Phosphonate, 42 0.92 2.17 Stretched chain

From the above results, it follows that the graft polymer according tothe invention allows for a grafting onto a metal oxide, wherein D/2 foradjacent polymers is substantially smaller than the Rg value of theindividual polymers. Accordingly, the adjacent polymers are forced bytheir mutual steric hindrance to adopt a stretched chain conformation.Consequently, the adjacent graft polymers according to the inventiontogether form a brush structure with the concomitant advantages such asa better shielding of the metal oxide surface.

FIG. 4 shows a metal oxide particle 40 of rutile, which is a type ofTiO2 crystal that is preferably used in the invention. The length of theparticle is approximately 140 to 180 nm. The width and height of theparticle is approximately 30 to 35 nm. The larger flat surfaces 42 areespecially suitable for grafting polymers onto according to theinvention. When grafted onto surfaces 42, the free tail ends of adjacentgrafted polymers have a limited distance to each other which forces themto adapt a stretched chain conformation, as long as the relationship isfulfilled that D/2 is smaller than Rg.

The invention claimed is:
 1. Graft polymer suitable for grafting onto ametal oxide surface, wherein the polymer comprises an addition polymerhaving a first and a second end, and a first moiety comprising aterminal phosphonate group, which first moiety is bonded to the firstend, and wherein the terminal phosphonate group is capable ofaccomplishing an attachment of the graft polymer onto the metal oxidesurface, wherein the polymer has a second moiety comprising a terminalaliphatic group, which second moiety is bonded to the second end via athiocarbonylthio (—SC(═S)—) group, wherein the terminal aliphatic groupis a linear alkyl group chosen from n-butyl up to n-dodecyl, wherein theaddition polymer is a polystyrene, polyisoprene, polyacrylonitrile,polymethacrylate, ABS, SAN, or a combination thereof.
 2. Graft polymeraccording to claim 1, wherein the addition polymer comprises a linearchain of carbon atoms.
 3. Graft polymer according to claim 1, whereinthe phosphorus atom P of phosphonate group is bonded to the first endvia a carboxylate group.
 4. Graft polymer according to claim 3, whereinthe carboxylate group is bonded to the first end via a —C(CH3)(CH3)-group.
 5. Graft polymer according to claim 1, wherein the second moietyis bonded to the second end via a tristhiocarbonate (—SC(═S)S—) group.6. Graft polymer according to claim 2, wherein the linear chain ofcarbon atoms comprises 10 to 50 carbon atoms.
 7. Graft polymer accordingto claim 3, wherein the phosphorus atom P of phosphonate group is bondedto the first end via a methylene carboxylate group (P—CH2-O(O═)C—). 8.Method of grafting a multitude of individual graft polymers according toclaim 1 onto a surface of a metal oxide, comprising the steps of:dissolving the individual graft polymers in an appropriate solvent;applying the formed solution onto the surface of the metal oxide;allowing the individual graft polymers to attach to the surface, whereinthe phosphonate group attaches to the metal oxide surface during themethod in such a way that the obtained multitude of grafted polymersonto the surface of the metal oxide comprises at least one group ofadjacent polymers that have a stretched chain conformation wherein theadjacent stretched chains have a substantially parallel orientation,such that the adjacent polymers within said group together form a brushstructure.
 9. Method according to claim 8, wherein the graft polymersthat are grafted onto the metal oxide surface, have a smallpolydispersity index (PDI) which is the ratio of the weight averagemolecular weight (Mw) and is the number average molecular weight (Mn),the PDI value being between 1 and
 2. 10. Method according to claim 9,wherein the PDI value is between 1 and 1.5.
 11. Method according toclaim 8, wherein the group of polymers forming a brush structure, havean average distance D on the metal oxide surface between adjacentpolymers, wherein D/2 is smaller than the average radius of gyration Rgof a random coil conformation of the individual grafted polymers. 12.Method according to claim 11, wherein D/2 is 70% of the average radiusof gyration Rg or smaller.
 13. Method according to claim 8, wherein thegroup of adjacent polymers forming a brush structure, are grafted onto anon-spherical metal oxide surface.
 14. Method according to claim 8,wherein the metal oxide is diamagnetic.
 15. Method according to claim14, wherein the metal oxide is titanium dioxide.
 16. Method according toclaim 8, wherein the metal oxide is in the form of a particle in therange of 20 to 200 nm.
 17. Method according to claim 16, wherein themetal oxide is a rutile nanoparticle.
 18. Method according to claim 8,wherein the metal oxide is in the form of a macroscopic sheet, such thatthe grafted polymers form a coating layer.